1. Field of the Invention
The present invention relates to a fuel cell comprising a membrane electrode assembly having an electrolyte and electrodes disposed on respective opposite sides of the electrolyte, and a pair of metal sheet separators sandwiching the membrane electrode assembly, the separators having reactant gas passages for supplying reactant gases to the surfaces of the electrodes of the membrane electrode assembly.
2. Description of the Related Art
Solid polymer electrolyte fuel cells employ an ion exchange membrane (electrolyte) comprising a solid polymer ion exchange membrane (proton exchange membrane). A membrane electrode assembly comprises an anode and a cathode, each made up of an electrode catalyst and a porous carbon sheet, that are disposed in confronting relation to the opposite sides of the ion exchange membrane. The membrane electrode assembly is sandwiched between separators (bipolar plates), making up a unit cell. A predetermined number of such unit cells are stacked for use as a fuel cell stack.
When a fuel gas, e.g., a gas mainly containing hydrogen (hereinafter referred to as “hydrogen-containing gas”) is supplied to the anode, the hydrogen in the gas is ionized on the electrode catalyst and moves through the ion exchange membrane to the cathode. Electrons produced while the hydrogen is in motion are supplied to an external circuit, which uses the electrons as an electric energy in the form of a direct current. Since the cathode is supplied with a gas mainly containing oxygen or air (hereinafter referred to as “oxygen-containing gas”), for example, hydrogen ions, electrons, and oxygen react with each other on the cathode, producing water.
In the fuel cell stack, the separators have, defined within their surfaces, a fuel gas passage for passing a fuel gas therethrough in facing relation to the anode and an oxidizing gas passage for passing an oxygen-containing gas therethrough in facing relation to the cathode. Coolant passages for passing a coolant therethrough are defined between the separators, the coolant passages extending along the surfaces of the separators.
Generally, the fuel gas passage and the oxygen-containing gas passage (hereinafter referred to as “reactant gas passages”) and the coolant passages are in the form of a plurality of passage grooves defined in the surfaces of the separators and extending from passage inlets to passage outlets which extend in the direction in which the separators are stacked. The passage grooves include straight grooves and folded grooves.
If the passage grooves are connected to passage inlets and outlets which comprise small openings, then buffer areas need to be provided around the passage inlets and outlets in order to supply fluids, including the fuel gas, the oxygen-containing gas, and the coolant uniformly into the surfaces of the separators along the passage grooves. If the passage grooves extend parallel to each other, then generated water tends to stay in particular passage grooves, and cannot be discharged efficiently therefrom.
There is known a fuel cell separator as disclosed in Japanese laid-open patent publication No. 8-222237, for example. According to the disclosed arrangement, as shown in FIG. 16 of the accompanying drawings, a separator sheet 1 comprises a thin metal sheet and has a number of embossed or dimpled protrusions 2, 3 formed on its face and back surfaces at spaced intervals of several mm.
The separator sheet 1 has a fuel gas inlet 4a and a fuel gas outlet 4b that are defined in respective opposite side edges thereof, and an oxygen-containing gas inlet 5a and an oxygen-containing gas outlet 5b that are defined in respective opposite upper and lower edges thereof.
The protrusions 2 project from one surface 1a of the separator sheet 1, defining a fuel gas passage 6 therebetween which communicate with the fuel gas inlet 4a and the fuel gas outlet 4b. The protrusions 3 project from the other surface 1b of the separator sheet 1, defining an oxygen-containing gas passage 7 therebetween which communicate with the oxygen-containing gas inlet 5a and the oxygen-containing gas outlet 5b. 
A fuel gas supplied from the fuel gas inlet 4a to the surface 1a of the separator sheet 1 flows through the fuel gas passage 6 continuously extending between the protrusions 2, and is supplied to an electrode (not shown). The fuel gas which is not used is discharged into the fuel gas outlet 4b. 
An oxygen-containing gas supplied from the oxygen-containing gas inlet 5a to the surface 1b of the separator sheet 1 flows through the oxygen-containing gas passage 7 continuously extending between the protrusions 3, and is supplied to an electrode (not shown). The oxygen-containing gas which is not used is discharged into the oxygen-containing gas outlet 5b. 
On the separator sheet 1, the protrusions 2, 3 project on respective different sides thereof, and the fuel gas passage 6 and the oxygen-containing gas passage 7 are defined by the protrusions 2, 3 which are independent of each other. Therefore, the fuel gas and the oxygen-containing gas tend to fail to flow uniformly in the surfaces 1a, 1b, producing areas in the surfaces 1a, 1b where the fuel gas and the oxygen-containing gas are not sufficiently supplied to the fuel gas passage 6 and the oxygen-containing gas passage 7. Therefore, it is difficult to supply the fuel gas and the oxygen-containing gas uniformly to the surfaces of the electrodes, and generated water is liable to be trapped by and stay around the protrusions 3, etc. and cannot smoothly be discharged.
Coolant passages may be defined between the protrusions of the separator sheet 1. If there are areas where the coolant does not smoothly flow through the coolant passages, then the electrodes are not cooled sufficiently, resulting in a higher temperature and a lower humidity which lead to an increased resistance overpotential.
When the electrodes are not cooled sufficiently, the distribution of electric energy generated in the generating surface of the fuel cell is likely to become irregular and the durability of the fuel cell tends to be lowered due to an increase in the temperature of the ion exchange membrane. If an increased amount of coolant is supplied to prevent the above performance reduction, then the overall fuel cell system suffers a drop in the efficiency and an increase in the pressure loss.
There is also known another fuel cell separator as disclosed in Japanese laid-open patent publication No. 2000-182631, for example. The disclosed separator is made of gas-impermeable dense carbon and has a plurality of convexities on both surfaces thereof which define reactant gas passages. The reactant gas passages have bent portions having channel-shaped bent ribs which define a plurality of equally spaced grooves for allowing reactant gases to flow smoothly along the bent portions.
However, since the separator is made of dense carbon, it has low toughness, is less resistant to vibrational fracture, and has a considerably large thickness. Therefore, fuel cells incorporating the disclosed separator cannot be reduced in overall size and weight.